I guess for me it would depend on what type of "group" you were talking about.

For choirs, hands down my favorite is James Hall & Worship and Praise. Females tend to be the dominant soloists in that group.

My favorite guy group of all time is Commissioned. They were showcasing awesome, tight vocal harmonies before they became commonplace. They also provided inspiration for male groups such as Boys 2 Men and N Sync.

My favorite female group is Witness. They started out as the female version of Commissioned but developed their own style. Like Commissioned, their style was very unique and their lyrics were entirely scriptural, which made their songs all the more beautiful.

As for ensembles one of my favorite is Jonathan Nelson & Purpose. They have an awesome sound and Jonathan Nelson is definitely an up and coming super songwriter.